In the book they said about 1/2 the kids with low processing speed had social difficulties. They also talk about how many of them have language difficulties, which of course predispose them to social interaction difficulties. A lot of it is very subtle. It is stuff like taking longer to pick up on social cues, stilted awkward interactions since it can take longer for them to formulate responses, delayed reactions to jokes or sarcasm which seems a bit off to peers.